Purpose:High-Z materials are often encountered in cancer patients undergoing radiation treatment. Due to electron transport and saturation of CT number versus density, it is difficult for treatment planning system (TPS) to take account of dose perturbation. The purpose of this study is to measure dose perturbations and to verify dose calculation accuracy by a TPS with high-Z.

Methods:A phantom was created with high-Z placed at 1.5 cm depth. Dose measurements were made using plane-parallel ion chamber and EBT2 film. The measurement was carried out for a 6MV photon for square field sizes of 3Χ3, 5Χ5 and 10Χ10 cm2 for various high-Z and variable thickness (Pb 0.1 mm, 0.5 mm, Sn 0.1 mm, 0.5 mm and Cu 0.5 mm) Dose calculation was performed using AAA algorithm with 1mm calculation grid. Forward dose perturbation factor (FDPF) was defined as the ratio of the dose with high-Z to the dose without high-Z.

Results:Ion chamber measurement shows that FDPF increase and reaches a value of to 1.0, as thickness and atomic number of high-Z decreases. At 1.5 cm or more from high-Z, the difference of FDPF between ion chamber and TPS was within 2 %. At 0, 0.2 and 0.5 cm from high-Z, ion chamber measurement were lower than TPS by an average of 12, 8 and 4 %, respectively. The measured and calculated difference increases, as thickness and atomic number of high-Z increases. There was no field size dependency except surface of high-Z. The difference of FDPF between ion chamber and EBT2 film was up to 4 %.

Conclusion:TPS calculation overestimate doses near high-Z interfaces. The difference of FDPF between ion chamber and TPS decreases, as the distance from high-Z increase.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by a grant from the Japan Society
for the Promotion of Science (JSPS) Core-to-Core Program
(Grant No. 23003).